Coating composition for polyethylene films comprising EPR and polybutene

ABSTRACT

An improved polymeric composition comprising a blend of polybutene-1 with an ethylene-propylene copolymer which, when combined with a shrinkable, thermoplastic substrate, produces a laminate having improved abuse and tear resistance and reduced self-adherence.

This is a division of application Ser. No. 428,541, filed Dec. 12, 1973,now U.S. Pat. No. 3,891,008.

FIELD OF THE INVENTION

This invention relates to blended compositions for producingthermoplastic films and like articles. More specifically, this inventionrelates to a layer composition for multilayer thermoplastic films whichare created by extrusion coating or other process for lamination whichwill produce oriented, heat-shrinkable films.

BACKGROUND OF THE INVENTION

The use of heat-shrinkable thermoplastic films in many packagingapplications is well known. Usually a product is enclosed in a film,sealed, and then heat is applied thus shrinking the film snugly aboutthe product. One of the most useful and satisfactory thermoplasticmaterials for packaging film is polyethylene. In the prior art, thereare a number of methods to convert raw polyethylene into a suitable,heat-shrinkable packaging film. One such method is disclosed in U.S.Pat. No. 2,855,517 issued to W. C. Rainer et al. on Oct. 7, 1958.Another method is taught by U.S. Pat. No. 3,022,543 issued to W. G.Baird, Jr. et al. on Feb. 27, 1962. In the Baird method, polyethylene iscontinuously extruded in the form of a tube, drawn, irradiated withelectrons to crosslink the molecules, heated, and stretched again byapplying internal pressure to the tubing. The resulting film usually hasgood heat shrinking properties but relatively low tear strength. Once atear has begun it tends to rapidly propagate during the heat shrinkingprocess resulting in destruction of the film. One method of overcomingthis low tear strength is disclosed in U.S. Pat. No. 3,754,063 issued toHenry G. Schirmer on Aug. 21, 1973. In the Schirmer patent, a method ofextrusion coating crosslinked polyethylene with a layer comprising amixture of isotactic polypropylene, polybutene-1, and atacticpolypropylene to improve the tear strength, bursting resistance, andheat sealing proprties of polyethylene is taught. However, whilepackaging materials, particularly bags for food products, made from thecoated polyethylene material disclosed in the Schirmer patent aregenerally quite satisfactory, it was discovered that it was necessary todust bags and films made from such coated polyethylene with cornstarchin order to prevent them from sticking together during storage. Thisdusting proved to be a time consuming production step and the dust,while not harmful, is an annoyance in handling the bags. Thus, it is anobject of the present invention to provide a superior packaging materialwhich does not require excessive dusting.

Another object of the present invention is to provide a coatedpolyethylene film product having improved abuse resistance andorientation properties with a reduced stickiness or tackiness.

Still another object is to provide a film which is easier to handle,stack, and use when made into pouches and bags.

The accomplishment of these and other objects will become obvious fromthe description of the present invention which follows.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a blended coating compositionfor thermoplastic substrates which comprises a major portion which is acopolymer of ethylene and propylene and a minor portion which ispolybutene-1. Preferably, the copolymer is present in the coatingcomposition in the range of 60% to 95% by weight and the polybutene-1 ispresent in the range of 5% to 40% by weight. Furthermore, in theaforementioned preferred ranges, the polypropylene constituent of thecopolymer is substantially greater than the ethylene constituent, theethylene constituent preferably being present in the range of 2.5% to 4%by weight but it may be as high as 10% by weight. The optimum coatingcomposition has been found to be one in which the polybutene-1 comprisesabout 25% by weight of the blended coating and the ethylene - propylenecopolymer comprises about 75% by weight of the blend with the ethylenepart of the copolymer being in the range of 3.0% to 3.5%.

In another aspect, the present invention is a blended composition whichmay form a layer in a multilayer thermoplastic film which comprises asubstrate layer of an oriented thermoplastic material in combinationwith a layer comprising the blended mixture of the present invention. Instill another aspect, the present invention is a method of improving thetear and abuse resistance of and reducing the tackiness or stickiness ofa multilayer, oriented thermoplastic film by coating the substrate witha blended mixture comprising a major portion of ethylene - propylenecopolymer and a minor portion of polybutene-1.

DESCRIPTION OF THE DRAWINGS

The present invention may be better understood by reference to thefollowing detailed description and drawings in which:

FIG. 1 is a fragmentary cross section of a multilayer thermoplastic filmhaving a layer comprising the blended composition according to thepresent invention;

FIG. 2 is a fragmentary multilayer thermoplastic film according to thepresent invention with an intermediate adhesive layer;

FIG. 3 is a bag produced from tubular, multi-layer thermoplastic filmprepared according to the present invention;

FIG. 4 is a graphical representation of two curves, one curve beingorientability of multilayer films according to the present inventionversus the coating composition proportions and the other curve beingabuse resistance versus coating composition proportions; and,

FIG. 5 is a schematic representation of the process for making amultilayer film according to the present invention.

PREFERRED EMBODIMENT

The process for making the preferred embodiment of the present inventionwill be first briefly described and then the test results which led tothe discovery of the preferred embodiment will be explained. The processis essentially the same as that disclosed in U.S. Pat. No. 3,754,063,mentioned above, and a brief description of the process is included herefor better understanding of this invention.

Turning now to FIG. 5, extruder 9 is shown from which extrudate 10 intubular form emerges from circular die 11. This extrudate may bepolyethylene or may be a copolymer of ethylene - vinyl acetate with 2%to 4% vinyl acetate. As the extrudate moves downwardly, it is cooled bycooling ring 12 before it is collapsed and passed around pinch rolls 13.The now flattened tubing enters irradiation vault 14 having shielding 15around it and the flattened tubing is threaded through festooning rolls17 in order to make a number of passes through the electron beam whichemanates from electron accelerator 16. This electron irradiation inducescrosslinking of the long chain polyethylene molecules. After leaving theirradition vault 14, the flattened tubing passes through pinch rolls 18where it is thereafter opened as inflated tubing 20 to pass throughcircular die 22 which is fed from extruder 25. Die 22 will either in oneembodiment, coat inflated tubing 20 with the blend of polybutene-1 andethylene-propylene copolymer or, in another embodiment, die 22 will coatan adhesive comprising an ethylene-vinyl acetate copolymer with 7% to20% vinyl acetate onto inflated tubing or substrate 20. Vacuum chamber21 which surrounds the inflated tubing 20 has vacuum line 29 attachedthereto and the vacuum chamber 21 is used to suck in the extrudedcoating 23 so that it adheres uniformly to the tubing 20 without anybubbles of air being trapped between the tubing and the coating. Leavingthe die 22 the now coated substrate 24 passes through cooling ring 30before it is collapsed in pinch rolls 31. If no adhesive has beenemployed as an intermediate layer the collapsed tubing could beimmediately fed to the hot water bath tank 44 for orientation. However,when the adhesive is applied the coated substrate will be fed againthrough pinch rollers 41 and opened as coated substrate 40 passesthrough vacuum chamber 37 on its way to coating die 32 which is fed byextruder 36. Extruder 36 will feed the molten blended coating mixture ofthe present invention to die 32 where the coating 34 is again suckedonto the coated substrate 40. The tubing 33 now has an adhesive layerplus a coating layer, and as it passes through cooling ring 35 thecoating is brought below its melting point so that the tubing can becollapsed by pinch rolls 42. The collapsed tubing is threaded aroundrolls 43 into hot water bath 45 within tank 44. The water 45 ismaintained at or close to its boiling point of 212°F which is below themelting point of the irradiated polyethylene substrate but is within thetemperature range in which the substrate can be oriented. As the coatedtubing moves upwardly and out of the heated bath it expands under theinternal air pressure maintained in the bubble 46 and is stretchoriented into film thickness. Thereafter it is collapsed by rollers 47,passed through rolls 48 and wound up on wind up roll 49.

Following the procedure as described with reference to FIG. 5, anethylene vinyl acetate copolymer containing approximately 31/2% vinylacetate is fed into the hopper of extruder 9 which is a 21/2 inch Hartigextruder which is operated at the following temperatures: rear zone300°F; mid barrel 330° to 350°F; front barrel 350°F; adaptor 350°F; anddie 350°F. The extrudate or extruded tubing leaving the die has a layflat width of approximately 2 7/16 inches and a wall thickness of 0.018inches. In the irradiation vault 14, this tubing receives a dosage ofabout 6.3 mr. In the preferred embodiment, the irradiated substrate 20continues to a die 22 where the adhesive coating is applied byextrustion and there a crosshead die is fed by a Prodex extruder whereinall zones of the extruder, the adaptor, and the die are kept at 450°F.The preferred coating is an ethylene vinyl acetate copolymer containingabout 81/2% vinyl acetate and the coating thickness is approximately0.003 inches. After receiving the adhesive coating, the tubing 40 passesthrough a second crosshead die 32 to receive a coating according to thepresent invention. To prepare the blend before it is fed into the hopperof extruder 36, pellets of ethylene-propylene copolymer containing 3.1%ethylene are blended in a Banbury mixer with pellets of polybutene-1 inthe ratio of 75% copolymer by weight and 25% polybutene-1 by weight. Allzones of the extruder range between 450° and 480°F and the resultingcoating thickness is 0.006 inches. Water is impinged on the heatedtubing at 62°F by the cooling rings 30 and 35 at their respectivelocations. To reach the final film thickness and become an orientedfilm, the tubing is fed into the hot water bath 44 which is maintainedbetween 210° and 212°F and the internal pressure of the bubble 46 issufficient to stretch the tubing to a film which has approximately 8inches of lay flat width and has a wall thickness of 2.2 mils.

Tubing which has been produced in the foregoing manner without anintermediate adhesive coating will have a cross sectional appearance asshown in FIG. 1 where laminate 3 is shown with substrate 1 havingcoating 2 adhered thereto by extrusion coating. When the intermediateadhesive layer is employed, the cross section of film prepared by theforegoing described manner will have an appearance and cross section asshown in FIG. 2 where laminate 3' is composed of substrate 1 havingadhesive coating 4 with extrusion coating 2 applied thereto.

The tubing which has been prepared as described hereinabove and rolledupon wind up roll 49 can be converted into bags by sealing the bottom ofthe bag by heat sealing across the entire width of the tubing and thensevering the tubing at the desired bag length. In FIG. 3, a prepared bag5 is shown with its bottom being sealed off by the continuous heat seal7 and the mouth 6 being prepared by severing the tubing from thepreviously formed bag.

The following tests were run under commercial conditions using bags madefrom tubing which was produced substantially as has been heretoforedescribed. Each test was made in a commercial plant and involved thepackaging of dressed poultry such as turkeys or baking hens. Dependingon the size of the birds to be packaged the width of the bags will rangefrom 8 to 16 inches and the length will range up to 24 inches. Topackage a bird, it is inserted into a bag, the bag is evacuated by avacuum nozzle with the bag neck held around the nozzle, the bag neck isthen twisted and closed with a metal clip. The closed bags are thenpassed through a hot water shrink tunnel which is operated in the rangeof 205° to 208°F whereupon the bag shrinks skin tight around the bird.

This type of packaging for the test results reported below was carriedout at various poultry packers in actual production runs to determinethe abuse resistance of the bags. In each test run, the number of bagstested; and hence, the number of birds packaged was usually in the orderof two thousand packages. When being filled, evacuated, clipped, hotwater shrunk, and moved at production speeds by conveyors the bagsloaded with relatively heavy turkeys undergo quite severe abuse. Recordswere kept of the number of failures and a failure was considered tooccur when the package integrity is ruined such as by a tear or cut intothe bag. In each test run standard bags were also used and the standardbag against which the abuse resistance results were compared is astandard bag made according to the process described in theaforementioned U.S. Pat. No. 3,754,063.

Before listing the specific test examples the graph of FIG. 1 should beexplained. The abscissa of the graph represents the proportion ofcopolymer to polybutene-1. The percentage of copolymer increases towardsthe left and percentage of polybutene-1 increases towards the right. Theethylene-propylene copolymer used for this graph has an ethylene contentin the range of 3.0 to 3.5 weight percent. The two curves in the graphare designated O--O and A--A for orientability and abuse resistancerespectively. Orientability is also called "rackability" in the art andthis is a measure of the ability of a tubular film to be oriented,particularly to be oriented in the trapped bubble technique as describedwith relation to FIG. 5. Abuse resistance was determined in actualpacking house tests as described above. The units on the two ordinatesin FIG. 1 are relative figures of merit and range from 1 to 5 with 1being unacceptable or very poor; 2 being poor and generallyunsatisfactory; 3 being average and acceptable; 4 being above average;and 5 being excellent. For example, an orientability of 1 means that aorienting bubble cannot be sustained for production purposes; and, ascan be seen in FIG. 1, it is not until the polybutene-1 content has beenincreased to about 20% that there becomes any significant increase inthe orientability of the blended mixture. As the copolymer/polybutene-1ratio goes from 80/20 to 70/30 a marked increase in the orientability ofthe blend is observed. On the other hand, the abuse resistance arisessomewhat slowly and also significantly improves the 80/20 to 70/30range. A crossover point occurs at a copolymer/polybutene-1 ratio of75/25 and this blend ratio provides an optimum combination oforientability and abuse resistance. Outside of the optimum range shownin FIG. 1 orientability becomes too poor as the copolymer is increasedbeyond 80% to make feasible continuous production; and, when theproportion of a copolymer is less than 70% the abuse resistance dropsoff significantly and resulting packages are not commerciallyacceptable. Thus, the blend ratio of copolymer/polybutene-1 has beendiscovered to reach its optimum value at a 75/25 ratio within theoptimum ratio range of 70/30 to 80/20.

It should be noted that changing the ethylene content in the copolymerwould change the optimum range. For example, it has been found thatusing an ethylene-propylene copolymer of 2.5% ethylene, the optimumblend using the procedures descrived above was 66% copolymer and 34%polybutene-1. On the other hand, if the ethylene content of thecopolymer is increased to 4% it could be expected that the percentage ofthe copolymer would be increased in order the obtain the optimum ratio.However, ethylene-propylene copolymers having an ethylene content ofgreater than 4% are not readily available at the present time.

The results of the various tests conducted with the different blendratios are briefly summarized in the examples set forth below:

Example I

Bags were prepared by the process described hereinabove wherein theethylene-propylene copolymer content was 66% and the polybutene-1content was 34%. A total of 1,500 of these bags were used to packagewhole turkeys. Of this total, 29 rejects were produced with 19 of thesebeing related to bag material. A total of 2,250 standard bags were usedto package whole turkeys in the same production run and produced 29rejects of which 19 were related to bag material failure. Thus, the testbag produced 1.3% overall rejects comparted with 0.75% for the standardbag. While not superior to the standard, this test indicated that thetest bags were capable of approximately the same performance.

Example II

Test bags having a blend ratio of 75%/25% copolymer/polybutene-1 wereprepared according to the process described above. 2,000 turkeys werepackaged in the test bag and 2,000 were packaged in the standard bags.The test bags had 0.3% overall rejects compared with 0.8% rejects forthe standard bag. Sheen and/or gloss was noticably more attractive onthe test bags; and, after freezing 11 standard bags were torn and notest bags were torn. Thus, the bags in this test demonstrated superiorabuse resistance to the standard.

Example III

Test bags having a blended coating of 68% ethylene-propylene copolymerwith 32% polybutene-1 were prepared according to the above describedmethod. In a test involving 2,144 bags, the test bags produced 1.3%rejects due to bag failures while the standard bag 1.2% rejects for thesame reason. Thus, the test bags were favorably comparable to thestandard bags in this test.

Thus, as the foregoing examples demonstrate, the blended mixture of thepresent invention has optimum properties when the weight percentcopolymer/weight percent polybutene-1 is in the range of 70/30 to 80/20for a copolymer having an ethylene content of 3.0 to 3.5%. However, alaminate can be made with a copolymer percentage as low as 60% and ashigh as 95% when the ethylene content of the copolymer is in the 2.5% to4% range but in the extremes of such a range there is a sacrifice inabuse resitance and in orientability. Furthermore, if the percentage ofethylene in the copolymer is increased, the proportion of copolymer maybe increased and an ethylene content in the copolymer in the rangebetween 0.1 and 10% is within the scope of the present inventionalthough, at the present, copolymer having greater than 4% ethylene isnot available in commercial quantities. In addition, the blend of thepresent invention is not limited to a single process such as theextrusion coating process described above as it may be cast upon thesubstrate or may be made as a self-supporting film which may beadhesively bonded to a substrate if desired.

The substrate of the present invention is preferably an ethylene-vinylacetate copolymer having a vinyl acetate content between 0 to 5%,preferably between 2% to 4% by weight, and which has been crosslinked byirradiation in the dosage range of 2 to 7 mr. Higher dosages may be usedbut are not economical. As the substrate is extruded the cooling waterwhich impinges upon it is preferably in the range between 40° to 70°F.The preferred adhesive to bond the ethylene-propylene/polybutene-1 layerto the substrate is an ethylene-vinyl acetate copolymer having 7% to 20% vinyl acetate and preferably about 81/2% by weight.

The preferred embodiment of the present invention which is a blendedthermoplastic composition having 70% to 80% by weight of anethylene-propylene copolymer having 3.0% to 3.5% by weight of ethyleneand 30% to 20% by weight of polybutene-1 provides either aself-supporting film or a layer or coating for a substrate which can beoriented in hot water, i.e. water from about 160° to 212°F, and whichhas a unique and novel combination of properties which cannot beachieved with the constituents of the blend alone. For example, theblend provides a film or coating with gloss and without tackiness or theneed to dust the film or coating plus toughness and orientabilitythereby giving a combination of strength and softness not heretoforeavailable. In addition, the ability of films made from blends accordingto the present invention to be oriented from hot water results insignificant commercial advantages as the need for relatively inefficientradiant or hot air heat sources is eliminated.

The thickness of a film or a layer as extruded from the blendedcomposition of the present invention may range from 2.0 to 13.0 mils andwhen stretched at the preferred ratio of about 12 to 1, thicknesses inthe range from 1/4 mil to over 1.0 mil result.

Having described my invention,

I claim:
 1. A blended thermoplastic coating composition consistingessentially of:a. a major portion which comprises a copolymer ofethylene and propylene, the ethylene constituent of said copolymer beingpresent in the range of 2.5% to 10% by weight; and, b. a minor portionwhich comprises polybutene-1.
 2. The coating composition of claim 1wherein said major portion is in the range of 95% to 60% by weight; theethylene constituent of the copolymer is in the range from 2.5% to 4.0%;and, the minor portion is in the range of 5% to 40% by weight.
 3. Thecoating composition of claim 2 wherein said major portion is in therange from 70% to 80% by weight and said minor portion is in the rangeof 30% to 20% by weight.
 4. The composition of claim 2 wherein saidmajor portion is about 66%; the ethylene content is in the range from2.5% to 3.0%; and the minor portion is about 34%.